Dextromethorphan is widely used as a cough suppressant and is considered to be safe enough to be sold over the counter. Although dextromethorphan may potentiate the analgesic effect of opiates in animals, clinical studies of combinations of dextromethorphan and opiates have generally been disappointing, and have failed to demonstrate this potentiating effect in humans. Three large multicenter randomized studies failed to show any benefit from the addition of dextromethorphan to morphine. The authors concluded “[t]hese results suggest that adding the NMDA antagonist, dextromethorphan, to opioids does not add any clinical benefit” and the addition of dextromethorphan to morphine “failed to enhance opioid analgesia.”